Lead-free CU-Zn alloy

ABSTRACT

A lead-free Cu—Zn alloy with improved machining properties compared to the alloy CuZn42, consisting of: 57-59.3 wt % Cu; 0.12-0.17 wt % Fe as a first alternative, or up to 0.06 wt % Fe and 0.3-0.7 wt % Mn as a second alternative; 0.03-0.1 wt % P; up to 1.0 wt % Sn; up to 0.1 wt % Pb; balance Zn together with unavoidable impurities, which are permitted up to 0.05 wt % per element, wherein the sum total of unavoidable impurities does not exceed 0.15 wt %; and wherein the following elements are tolerated up to the following specified contents: up to 0.03 wt % Ni, up to 0.05 wt % Al, up to 0.01 wt % Si, up to 0.01 wt % Cr.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to EP 20204628.0 filed Oct. 29, 2020 which is incorporated by reference herein

BACKGROUND

The present disclosure relates to a lead-free Cu—Zn alloy with improved machining properties compared to the alloy CuZn42.

Due to the small number of elements involved in the structure of the alloy, the alloy CuZn42 is a very simply structured brass alloy with a Cu content between 57.0 and 59.0 wt %. In principle, no other elements are involved in this alloy. Pb is tolerated up to a maximum of 0.2 wt %, Sn up to 0.03 wt %, Fe up to 0.3 wt %, Ni up to 0.02 wt % and Al up to 0.05 wt % together with unavoidable impurities. This alloy is a lead-free alloy that can be hot-worked very easily and is used, among other things, for the production of profiles as a semi-finished product. This alloy is the lead-free variant of the conventionally used alloy CuZn39Pb3. In the case of the CuZn39Pb3 alloy, the element lead is used primarily to improve machinability. Even though the alloy CuZn42 is lead-free, it is also used for machining, such as the production of turned parts, due to its α/β microstructure. However, the machinability of workpieces made from this alloy is limited. This means that the machining disadvantages caused by the alloy cannot be compensated by appropriate process parameters of a machine tool. This applies, for example, to machining processes with forming tools, in which the limits of the process parameters do not allow any respective leeway. In such cases, the machinability of such an alloy is unsatisfactory.

Even if the machinability for certain machining operations on workpieces made from this alloy is acceptable, it would be desirable if the machinability could be improved without having to use the Pb and Bi elements conventionally used for free cutting alloys to achieve the desired machinability, since these are classified as hazardous to health.

A Cu—Zn alloy with improved machining properties is known from EP 3 690 069 C1. This alloy contains 58-70 wt % Cu, 0.5-2.0 wt % Sn, 0.1-2.0 wt % Si, the balance being zinc and unavoidable impurities, with the sum total of the elements Sn and Si between 1.0 wt % and 3.0 wt %. The improved machinability without the use of the elements Pb and Bi is provided in this alloy by the Sn and Si contents. In the specified proportions, these elements are responsible for the formation of the ε phase, which phase is distributed as a microstructure in the alloy and thus promotes chip breaking. The Si contained in the alloy also leads to the formation of silicides, specifically together with the elements Al and Ni permitted in the alloy and/or Mn, which are regularly found in the alloy due to the usual use of recycled material. The Si content in this alloy can be 2.0 wt %. While silicides contained in the matrix are beneficial for some applications, especially when wear resistance requirements are present, silicides can be noticeable in machinability due to increased tool wear.

JP H03-253527 A discloses a Cu—Zn alloy for making electrical fuses. Multiple alloys are disclosed in this document. One of the disclosed alloys comprises 25-40 wt % Zn, 0.01-0.1 wt % P, 0.02-0.50 wt % Fe, the balance being Cu together with unavoidable impurities. According to alternative variants, the element Fe is replaced by the elements Co, Ni, or Mn, each in equal proportions. Thus, this alloy can have a Cu content of 59.4-75 wt %. The embodiments given for the Fe variant of this alloy have Fe contents between 0.03 and 0.12 wt. %. Other elements, with the exception of unavoidable impurities, are not permitted. This also applies to the Co, Ni and Mn variants. In addition to the aforementioned suitability as a material for manufacturing electrical fuses, this material also exhibits good strength and spring properties.

JP S59-153856 A discloses a Cu—Zn alloy with improved corrosion resistance for manufacturing heat exchangers. This alloy has the following composition: 25-40 wt % Zn, 0.005-0.070 wt % P, 0.05-1.0 wt % Sn, 0.005-1.3 wt % in total of 0.005-1.0 wt % Fe and/or 0.005-0.3 wt % Pb, the balance being copper plus unavoidable impurities. This Cu—Zn alloy also has a wide range in the Cu content, namely from 57.9-74.9 wt %. The grain size of this alloy is controlled to be no greater than 15 μm. The P content is provided to achieve the desired properties. Alloy 12 of Table 1 has the following composition: 63.8 wt % Cu, 0.1 wt % Fe, 0.07 wt % P and 1.0 wt % Sn, the balance being Zn. Mn is not an approved alloying element in this alloy.

SUMMARY

Proceeding from this background, one aspect of the present disclosure is to provide a lead-free Cu—Zn alloy with improved machining properties compared to the CuZn42 alloy, which proposed alloy has a simple structure and does not require any special manufacturing steps to achieve the desired good machinability.

This is achieved by a lead-free Cu—Zn alloy with improved machining properties compared to the CuZn42 alloy, consisting of (data provided in wt %):

-   -   Cu: 57-59.3%,     -   Fe: 0.12-0.17% (1st alternative) or Fe: up to 0.06% and Mn:         0.3-0.7% (2nd alternative),     -   P: 0.03-0.1%,     -   Sn: up to 1.0%,     -   Pb: up to 0.1%,     -   the balance being made up by Zn together with unavoidable         impurities, which are permitted up to 0.05% per element, wherein         the sum total of the unavoidable impurities does not exceed         0.15%,     -   wherein the following elements are tolerated up to the specified         contents:     -   Ni: up to 0.03%,     -   Al: up to 0.05%,     -   Si: up to 0.01%,     -   Cr: up to 0.01%.

Unavoidable impurities are permitted at 0.05 wt % per element, wherein the sum total of the unavoidable impurities does not exceed 0.15 wt %.

An alloy is considered to be lead-free if its Pb content does not exceed 0.1 wt %.

This alloy contains P, such that iron phosphides or manganese phosphides are formed, depending on the design of the alloy under the first alternative or the second alternative. The addition of phosphorus in the above proportions has a positive effect on casting, since phosphorus has a grain-reducing effect. This has a positive effect on the desired improved machinability. It is important in this context that the workpieces produced from the alloy have a fine grain without the need for additional measures, such as water quenching after pressing. Due to the composition of the alloy, the extruded semi-finished products already have a sufficiently fine grain. In addition, the finely dispersed phosphides contained in the matrix have a chip breaking effect. The chips produced during machining of a workpiece made of this alloy are significantly better than those produced during machining of the CuZn42 alloy due to their chip shape (crumbling chips or very short helical chips) and are very close to those produced during machining of the lead-containing machining CuZn39Pb3 alloy. It is essential that, despite the addition of phosphorus to form the phosphides, the strength properties of the products made from this alloy correspond to those of the comparative CuZn42 alloy. In addition to improved chip breaking, the surface finish obtained by machining is comparable to that obtained by machining the leaded predecessor alloy CuZn39Pb3. This is surprising to observe, since a lower surface finish was actually expected due to the phosphides distributed in the matrix and thus due to the more inhomogeneous matrix compared to the CuZn42 and CuZn39Pb3 alloys.

Likewise, the phosphides could not be expected to act as recrystallization inhibitors of the microstructure, particularly at elevated temperatures.

The P content is thus limited to 0.1 wt %. At higher P contents, grain coarsening of the phosphides occurs. This is disadvantageous for machining, as well as for certain surface treatments, such as polishing or coating. Although coarser phosphides improve the wear resistance of the workpiece produced from the alloy, this does not make up for the other disadvantages mentioned above. If the P content is less than 0.03 wt %, the advantageous properties described above occur only insufficiently or not at all.

The elements Fe and Mn are limited to the stated contents. If more Fe or Mn is used, this results in coarsening of the grain. Below the limits mentioned, the desired phosphides do not develop to a sufficient extent to achieve the machining-improving properties.

Sn may be involved in alloying and supports machinability. Sn is also advantageous in terms of the formation of the melt. The involvement of P makes the melt less viscous. Sn counteracts this effect. In addition, Sn in the melt can have a deoxidizing effect. Sn is incorporated into the alloy below the solubility limit in the mixed crystal. Otherwise, there is a risk that a Sn-containing γ-phase forms, which in turn has an embrittling effect on the alloy product. If Sn is used as an alloying element, the improved machinability is due, on the one hand, to the effect of phosphides described above and, on the other hand, to the mechanism of action of Sn. Both mechanisms of action complement each other. The involvement of Sn also helps in dry machining by forming Sn oxides, which reduces tool wear as the oxides transfer protectively to the tool surface. If a particularly simple alloy structure is desired, the principle of action based on Sn favoring machining can be eliminated. In such an embodiment, Sn is not used as an alloying element, but is only tolerated at a proportion of up to 0.1% by weight.

The tolerated accompanying elements do not adversely affect the improved machinability of a workpiece made from the alloy according to the present disclosure, at least not significantly. Recycled material can therefore be used to produce this alloy without having to accept disadvantages. For this purpose, recycling material from a preferably closed cycle is used, i.e. single-type recycling material is used. If recycling material is used in which, with regard to its composition, for example, one or more elements are not present or not present in the appropriate proportion, these elements can be added to the recycling material. This particularly applies to the element P, which is essential and generally not present when using conventional recycling material.

The zinc equivalent of the alloy according to the invention is approximately between 39 and 42, such that the alloy product has an α/β structure. The zinc equivalent is typically slightly lower compared to the CuZn42 alloy, with the consequence that a formation of α-phase is favored compared to the comparative alloy. This has positive effects on cold formability of products (workpieces) made from this alloy. This is of interest because the elements Fe and/or Fe and Mn have reduced the zinc equivalent only to such an extent that the cold formability is improved, but the good hot formability known from the CuZn42 alloy is still retained and, in addition, the phosphides already described above are formed.

A special feature of the alloy according to the present disclosure is that the improved machinability is based solely on the special composition of the alloy and that no additional measures, such as certain manufacturing or processing steps, are required. Therefore, the semi-finished products produced from the alloy can be produced using the usual manufacturing processes. This also has the advantage that for the processing of the semi-finished products to manufacture the final product, respective treatment steps to set specific strength and/or structural properties can be performed; these are therefore not yet consumed by the manufacturing process for producing the semi-finished products. In this context, it goes without saying that the improved machining properties are achieved without additional process steps, but that, if desired, they can be increased again after extrusion through special treatment steps such as cold forming to improve chip breaking and thus machinability.

DESCRIPTION OF FIGURE

FIG. 1 shows chip breaking for the alloy CuZn39Pb3 (left) and alloy 1 (right).

DESCRIPTION OF TESTS

Specimens were prepared from the alloys indicated below (data provided in wt %):

Cu Fe Mn P Sn Pb Zn CuZn39Pb3 57.2 2.8 bal. CuZn42 58 bal. Alloy 1 58 0.14 0.06 bal. Alloy 2 58 0.47 0.06 bal. Alloy 3 58.3 0.05 0.04 0.9 bal.

The specimens were prepared by casting and subsequent extrusion in bar stock, each with a diameter of 40 mm, which were stretched after extrusion. Machining tests were carried out on specimen pieces. Alloys 1 to 3 are alloys according to the present disclosure.

The machining tests were carried out uniformly for all specimens by external longitudinal turning at a cutting speed of 200 m/min, made with a depth of cut of 1 mm and a feed of 0.1 mm.

The results of the tests were rated in the form of indices from 0 to 100. In this system, the comparison alloy CuZn42 receives the index 50 for the various cutting indexes. The higher the index, the better the result.

Chip shape, cutting force, tool wear, and surface quality resulting from the cutting were examined. The results of the tests are given in the following table:

Chip shape Cutting force Tool wear Surface finish CuZn39Pb3 80 40 60 70 CuZn42 50 50 50 70 Alloy 1 70 30 65 62 Alloy 2 70 40 70 66 Alloy 3 70 30 80 68

To illustrate the chip shape obtainable with the alloys according to the present disclosure, reference is made to FIG. 1. FIG. 1 shows in a comparison the chip breaking of the comparative alloy CuZn39Pb3 (left), which shows the desired chip breaking shape, and the chip breaking of alloy 1 (right). This comparison illustrates that the alloy according to the present disclosure achieves a significantly improved chip shape compared to the lead-free comparative alloy CuZn42. The chip shape of the machined alloys 2 and 3 correspond to the chip shape of alloy 1 shown in FIG. 1.

A somewhat higher cutting force is required for cutting the alloys according to the present disclosure. The reason for this is the phosphides contained in the alloy, which, however, are responsible for better chip breaking and thus also for the overall improved machinability. Regarding machinability, the chip shape is a relevant factor, such that in this regard the somewhat higher cutting force compared to the comparison alloys can be accepted.

It is important to note that the alloys according to the present disclosure have an improved tool wear index compared to the CuZn42 alloy. This was not expected.

The surface finish of the alloys according to the present disclosure substantially matches that which is achieved with the comparative alloys, such that no disadvantages, at least no noteworthy disadvantages, have to be accepted in this respect.

The mechanical strength values of the alloy according to the present disclosure are given in the table below:

Tensile Elongation Brinell Yield strength strength at hardness Rp 0.2 [MPa] [MPa] break % HBW CuZn39Pb3 155 401 46 87 CuZn42 149 459 44 100 Alloy 3 205 575 37 105 Alloy 1 155 595 46 122 Alloy 2 170 607 46 127

These mechanical properties of the semi-finished products made from the alloy according to the present disclosure, obtained from the specimens in the as-pressed condition, make it clear that they have a significantly improved tensile strength and higher hardness compared to the comparative alloys. The elongation at break is similar to that of the comparative alloys. The same applies to the yield strength, which is, however, significantly higher for alloy 3 than the yield strength determined for the comparative alloys. Despite the improved machinability, there is therefore no need to accept any disadvantages with regard to the mechanical strength values of the alloys according to the present disclosure. Instead, these are even improved.

Due to the positive structural properties that are already established during pressing, a semi-finished product made from the alloy can be used for a wide variety of applications.

While a number of aspects and embodiments have been discussed herein, those skilled in the art will recognize numerous modifications, permutations, additions, combinations and sub-combinations therefor, without same needing to be specifically explained in the context of this disclosure. The claims should therefore be interpreted to include all such modifications, permutations, additions and sub-combinations, which are within their true spirit and scope. The terms and expressions herein are used for description and not limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown or described, or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the invention has been specifically disclosed by certain embodiments and optional features, modification and variation of the concepts herein may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the claims. Whenever a range is given, all intermediate ranges and subranges, as well as all individual values included in the ranges given are hereby incorporated into this disclosure. When a Markush group or other grouping is used, all individual members of the group and all combinations and sub-combinations possible of the group are hereby individually included in this disclosure. In general, the terms and phrases used herein have their art-recognized meaning, which can be found by reference to standard texts, references and contexts known to those skilled in the art. The above definitions are provided to clarify their specific use in the context of the invention. 

1. A lead-free Cu—Zn alloy with improved machining properties compared to the alloy CuZn42, consisting of: Cu: 57-59.3 wt %; Fe: 0.12-0.17 wt % as a first alternative, or Fe: up to 0.06 wt % and Mn: 0.3-0.7 wt % as a second alternative; P: 0.03-0.1 wt % Sn: up to 1.0 wt %; Pb: up to 0.1 wt %; balance Zn together with unavoidable impurities, which are permitted up to 0.05 wt % per element, wherein the sum total of unavoidable impurities does not exceed 0.15 wt %; wherein the following elements are tolerated up to the specified contents: Ni: up to 0.03 wt %, Al: up to 0.05 wt %, Si: up to 0.01 wt %, Cr: up to 0.01 wt %.
 2. The Cu—Zn alloy of claim 1 according to the first alternative, wherein the Fe content is 0.13-0.15 wt %.
 3. The Cu—Zn alloy of claim 2, wherein the P content is 0.05-0.08 wt %.
 4. The Cu—Zn alloy of claim 2, wherein the alloy contains 0.8-1.0 wt % Sn and ≤0.04 wt % P.
 5. The Cu—Zn alloy of claim 1 according to the second alternative, wherein the Fe content is ≤0.04 wt % and the Mn content is 0.35-0.55 wt %.
 6. The Cu—Zn alloy of claim 5, wherein the P content is 0.05-0.08 wt %.
 7. The Cu—Zn alloy of claim 1, wherein the P content is 0.05-0.08 wt %.
 8. The Cu—Zn alloy of claim 1 according to the first alternative, wherein the alloy contains 0.8-1.0 wt % Sn and ≤0.04 wt % P. 